Today, people use digital networks to communicate in many different ways. Speech, text, World Wide Web pages and streaming live video are just a few examples. The ability to connect to a digital network at a particular physical location is one thing. The ability to connect to a digital network while remaining mobile is quite another. Wireless digital networks are popular and becoming more so. People value the convenience and freedom offered by a wireless network.
Radio is the base technology underlying most wireless digital networks. Each wireless network base station or access point, and each mobile device capable of communicating with the access point, is able to both transmit and receive radio signals. The radio signals are encoded with wireless network data.
It is in the nature of radio signals that signal strength fades with increasing distance from a transmitter. Each wireless network is designed to operate in a certain range of signal strengths, so that when the signal strength drops below a certain point, the quality of service provided by the wireless network drops dramatically. The geographical area served by a single wireless network access point is a wireless network cell. A wireless network provides service to larger geographical areas by dividing the larger area into cells, each with an access point. As a mobile communications device moves through the cells, it communicates with each cell's access point in turn.
One of the traditional problems for wireless network designers is deciding when a mobile device should handoff communications from one wireless network access point to another. The decrease in the quality of service provided to a mobile device by a wireless network access point is nonlinear, particularly near cell boundaries, and the radio environment is dynamic, so that analysis of the problem is not trivial. In practice, most horizontal handoff algorithms rely on a comparison of the relative signal strengths from candidate access points. A horizontal handoff is a communications handoff that occurs between wireless network access points of the same type within a single type of wireless network.
There are many different types of wireless network, each designed to meet different goals. For example, wireless networks complying with standards such as the General Packet Radio Service (GPRS) and Code Division Multiple Access (CDMA) are designed to provide a relatively low level of digital communications bandwidth (e.g., 64 kbps) over a wide geographical area (e.g., a metropolitan area), whereas wireless networks complying with other standards, such as the Institute of Electrical and Electronics Engineers' (IEEE) 802.11 series of standards and the European Telecom Standards Institute's (ETSI) High Performance Radio Local Area Network (HIPERLAN) series of standards, are designed to provide a relatively high level of digital communications bandwidth (e.g., 10 Mbps) but over a much smaller geographical area (e.g., a building or campus). These examples illustrate one set of wireless network design tradeoffs, i.e., bandwidth versus cell size. Other design considerations include how the radio spectrum allocated to each network will be shared among its users as well as user authentication and privacy measures.
Many geographical locations are served by multiple wireless network types, for example, CDMA and IEEE 802.11b. It is desirable for a wireless network user to be able to take advantage of the best features of each network type, for example, to be able to take advantage of a CDMA wireless network's wide service area and also to be able to take advantage of an IEEE 802.11b wireless network's high bandwidth where service is available. It is possible for a wireless network user to manually switch between wireless network types but it is further desirable that vertical handoff, i.e., handoff between different types of wireless network, become as transparent to a wireless network user as horizontal handoff within a wireless network is today. A future is envisioned in which a wireless network user is able to roam seamlessly between network types according to the wireless network user's needs, but there are several problems that need to be solved in order for that future to be realized.
Wireless network standards such as CDMA and IEEE 802.11b, are generally incompatible and are not focused on supporting vertical handoffs. Signal strengths in different wireless networks are not directly comparable. A level of signal strength that provides a good quality of service in one wireless network may result in a poor quality of service in another. New handoff decision criteria are required. Some prior art (e.g., Optimization Scheme for Mobile Users Performing Vertical Handoffs between IEEE 802.11 and GPRS/EDGE networks, Ylianttila et al.) discusses the use of quality of service criteria, such as bandwidth, for making vertical handoff decisions but it is generally silent on how to obtain quality of service measures in practice. There is a need in the art for a practical system and method to implement seamless vertical handoffs between wireless network types.
The invention provides such a system and method. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.